Basics of Hazards Evaluation Procedures

By Steve Lelewer, Bechtel Nevada

In 1985, the American Institute of Chemical Engineers (AIChE) established the Center for Chemical Plant Safety (CCPS). CCPS was tasked with the preparation of guidelines for hazard evaluation procedures. Guidelines for Hazard Evaluation Procedures was published by CCPS in 1985 and has become a primary reference for professionals in the hazards assessment field. A number of hazard evaluation procedures, including HAZOP, are presented in the CCPS guidelines.

In response to the catastrophic events that occurred at Bhophal, Flizborough, Seveso, and other smaller but deadly chemical plant incidents, the U.S. Congress instructed the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) to develop regulations to protect workers and the citizens in communities surrounding facilities that produce, utilize, or store significant quantities of highly hazard materials. In response, OSHA promulgated regulation CFR 1910.119, Process Safety Management for Highly Hazardous Materials, in 1992. Subsequently, EPA issued 40CFR67, Risk Management Program, in 1994.

Both regulations are intended to increase the level of safety at facilities where highly hazardous materials are found. OSHA’s focus is worker safety and EPA’s focus is on off-site impacts. Both regulations require that rigorous hazards assessments be performed for facilities that exceed the defined threshold quantities of hazardous materials. The OSHA standard requires that the process hazards analysis must address:

The HAZOP procedure is specifically accepted by both regulations as a methodology for hazards assessments.

The HAZOP assessment identifies hazards and operability deficiencies in the process or procedure studied. The basis for the HAZOP methodology is the study of the response of the system to deviations from design intent. The primary principal of the HAZOP is that several experts with varying backgrounds brought together in a series of structured, facilitated “brainstorming” sessions will perform a more rigorous evaluation than if each performed an independent review and their results were combined.

First, Gather Data

To initiate a HAZOP assessment, system data must be gathered. As a minimum, process flow diagrams, control logic diagrams, set-points, and design data such as pressure, temperature, flow, fluid composition, and materials of construction are needed. Physical configuration drawings may also be useful. In addition, a valuable benefit of the HAZOP methodology is its ability to readily address operating procedures and the even more critical start-up and shutdown operations that are often the cause of system failures.

After the system data is gathered, the next step is to identify the assessment team. The team should consist of engineers familiar with the details of the design, specialists in the field of the process being utilized, safety professionals, materials specialists, process operators, and maintenance personnel. Also critical to the HAZOP assessment process is the team leader; in many cases, a scribe is utilized to capture the information presented in the brainstorming session. As stated above, the multidisciplinary nature of the assessment team is crucial to the quality of HAZOP results. Equally important is the ability of the team leader to facilitate the session, keeping all participants involved and ensuring the integrity of the HAZOP.

Divide the Process into Nodes

After the team is assembled, it is recommended that they perform a physical walkdown of the process. In the design phase of a process, this may not be possible. When the HAZOP is convened, the team leader works with the team to divide the process into study nodes. These nodes are usually primary system elements such as tanks, pumps, and pipes. If the nodalization is too coarse, the quality of the HAZOP is diminished, while, if the nodalization is too fine, the HAZOP becomes cumbersome and costly. Although it is best to establish adequate nodalization up-front, it can be modified during the HAZOP session.

Once the nodalization is complete, the scope of the assessment is agreed upon by the study team. The team starts the deviation assessment under the guidance of the team leader. The HAZOP methodology has a somewhat standardized documentation form. There are a variety of formats but the information gathered is the same.

A number of software packages are available from companies that specialize in hazards and risk assessment. The software packages make documentation easier and may eliminate the need for a scribe. This can reduce the cost of the assessment, but some argue that having the team leader document the session reduces his effectiveness as a facilitator.

For organizations that do not perform HAZOPs regularly, many hazards and risk assessment consulting firms offer team leader and scribe support. If a solution to a deficiency is readily discernable during the HAZOP, then the solution is documented in the assessment report. If a solution is not apparent, then the issue remains open and responsibility for resolution is assigned.

For those issues where a potential deficiency is identified, the team performs a qualitative assessment as to the risk presented by this deficiency. Risk is a function of both probability of occurrence and consequence of failure. The higher the risk, the more critical the resolution. The establishment of “acceptable risk” is necessary and usually set at the corporate policy level where the legal liability also resides.

The HAZOP Vocabulary

The HAZOP methodology has an associated vocabulary:

Table 1: Qualitative Severity Ranking
Ranking Description
1 Catastrophic On-site fatality; >$1 million physical damage; process outage >1 month; significant off-site consequence (property damage, environmental impact, public health and safety)
2 High On-site lost-time accident; >$50,000 physical damage; process outage >1 week, and < 1 month; off-site alert or response required
3 Moderate On-site reportable injury; reportable environmental release; $10,000 to $50,000 physical damage; process outage > 1 day and < 1 week
4 Low Below reportable environmental impact; <$10,000 damage; process outage 1 day
5 None No consequence to work safety, public health, or environment

Rank Frequency of Occurrence
1 Once in six months
2 Once in a year
3 Once in 2 years
4 Once in 5 years
5 Once in lifetime of process

Risk Values
Severity/
Likelihood		 1 2 3 4 5
1 1 2 3 4 5
2 2 4 6 7 8
3 3 6 7 8 9
4 4 7 8 9 10
5 5 8 9 10 10

The work process of the HAZOP study is as follows:

  1. Select NODE
  2. Select PARAMETER
  3. Describe INTENTION
  4. Select DEVIATION
  5. Identify credible CAUSES
  6. List significant CONSEQUENCES
  7. List existing SAFEGUARDS
  8. Estimate SEVERITY, LIKELIHOOD, and, therefore, RISK
  9. Record RECOMMENDATION
  10. Next DEVIATION
  11. Next PARAMETER
  12. Next NODE
  13. Complete Study.

©1998. All Rights Reserved. A Publication of the National Hydrogen Association.
This material may not be reproduced in any form without permission.

Home Page • Return to NHA News Index